This happened to me before except i was running Vista x64 at the time. It turned out in my case that the power management option for my mouse was keeping it from sleeping, the box was checked next to allow the device to wake the computer. try unchecking those boxes in device manager for the mouse and possibly your keyboard.
(its funny how a mouse can keep a computer up too lol.....{cricket in background} is my mic on)

Thanks for the idea.
I had already unchecked the mouse and the network card (the most common culprit in sleep malfunctions)...the keyboard is what I use to rouse the PC from slumber, so it has to be allowed to wake the machine.
Besides, why would the keyboard misfire exactly every other time?

All true, but...
The settings for all these things have been consistent since I first started using the Win7 builds (lo, these many months ago), and sleep has worked flawlessly right up till the RC1 build install.
Sometime this week I hope to do a reinstall and see what happens.